1. Field of the Invention
This invention relates to a receiver which performs tuning by a digital designation of a receiving frequency in the case of receiving a broadcast of a desired broadcasting station.
2. Description of the Prior Art
In an AM/FM radio receiver of the type performing a tuning operation by changing at least the local oscillation frequency with a control voltage, there is no need of such a mechanical operation as turning a variable condenser as is required in prior art receivers. With such an arrangement that the abovesaid control voltage is generated by a sweep voltage generator to stop a frequency search upon reception of a desired receiving frequency, that is, a desired broadcasting signal, purely electronic control of tuning is possible, and if a digital setter for designating the receiving frequency and a comparator are provided, detection of the desired frequency and storing of the search operation (that is, tuning) be achieved automatically. In other words, a desired broadcasting station can be tuned only by changing the set value of the digital setter.
For the digital designation of the receiving frequency, it is possible to represent the broadcasting frequency directly with a BCD (Binary Coded Decimal) code. However, such a system requires 12 or 10 bits, and hence introduces complexity in the comparator, the frequency setter and so on.
Further, an electronic tuning operation is made possible by using, as the local oscillator, a voltage-controlled oscillator which changes its oscillation frequency with a control voltage. The control voltage may be produced as by a variable resistor. Furthermore, use, made of a voltage memory device, that is, an element which, has an integrating function and which when supplied with a positive or negative voltage, gradually increases or decreases its output. Further, the arrangement can be such that, upon removing the input voltage or causing it to fall to zero, the aforementioned element holds the present output voltage for a long time. In this way, a control voltage can easily be obtained which is continuously varied only by the application of such a positive or negative voltage and holds a desired value. In this case, however, simplification of means for applying the designating voltage to the voltage memory device and auto repeat of the tuning operation present problems.
Moreover, in the prior art receiver, muting is effected by a method of actuating a switching element at the level of an IF (intermediate frequency) signal to turn on and off an audio signal. This method is sufficient for manual operation or search-type auto tuning, but in a receiver which has a plurality of tuning functions or is capable of digital setting of a desired receiving frequency, such as an electronic tuning receiver, it is difficult to set muting only at the level of the IF signal. For example, in digital setting, where a plurality of broadcasting stations are met while searching from a station A to B, muting set only with the IF signal may be interrupted at the frequencies of the plurality of broadcasting stations in some cases.